Motor gasoline sulfur level regulations are expected to result in a need for the production of less than 50 wppm sulfur mogas by the year 2004, and perhaps levels below 10 wppm in later years. In general, this will require deep desulfurization of catalytically cracked naphthas (“cat naphthas”). Cat naphthas result from cracking operations, and typically contain substantial amounts of both sulfur and olefins. Deep desulfurization of cat naphtha requires improved technology to reduce sulfur levels without the loss of octane that accompanies the undesirable saturation of olefins.
Hydrodesulfurization is a hydrotreating process employed to remove sulfur from hydrocarbon. The removal of feed organosulfur by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfided supported and unsupported catalysts, especially those of Co/Mo and Ni/Mo. Severe temperatures and pressures may be required to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process.
Olefinic naphthas, such as cracked naphthas and coker naphthas, typically contain more than about 20 wt. % olefins. At least a portion of the olefins are hydrogenated during the hydrodesulfurization operation. Since olefins are high octane components, for some motor fuel use, it is desirable to retain the olefins rather than to hydrogenate them to saturated compounds that are typically lower in octane. Conventional fresh hydrodesulfurization catalysts have both hydrogenation and desulfurization activity. Hydrodesulfurization of cracked naphthas using conventional naphtha desulfurization catalysts under conventional startup procedures and under conventional conditions required for sulfur removal typically leads to a significant loss of olefins through hydrogenation. This results in a lower grade fuel product that needs additional refining, such as isomerization, blending, etc. to produce higher octane fuels. This, or course, adds significantly to production costs.
Selective hydrodesulfurization, i.e., hydrodesulfurizing a feed with selective catalysts, selective process conditions, or both, may be employed to remove organosulfur while minimizing hydrogenation of olefins and octane reduction. For example, ExxonMobil Corporation's SCANfining process selectively desulfurizes cat naphthas with little or no loss in octane number. U.S. Pat. Nos. 5,985,136; 6,013,598; and 6,126,814, all of which are incorporated by reference herein, disclose various aspects of SCANfining. Although selective hydrodesulfurization processes have been developed to avoid significant olefin saturation and loss of octane, H2S liberated in the process can react with retained olefins to form mercaptan sulfur by reversion. Such mercaptans are often referred to as “recombinant” or “reversion” mercaptans.
Sulfur removal technologies can be combined in order to optimize economic objectives such as minimizing capital investment. For example, naphthas suitable for blending into a motor gasoline (“mogas”) can be formed by separating the cracked naphtha into various fractions that are best suited to individual sulfur removal technologies. While economics of such systems may appear favorable compared to a single processing technology, the overall complexity is increased and successful mogas production is dependent upon numerous critical sulfur removal operations. Economically competitive sulfur removal strategies that minimize capital investment and operational complexity would be beneficial.
Consequently, there is a need in the art for technology that will reduce the cost of hydrotreating cracked naphthas, such as cat naphthas and coker naphthas.